Credentials of the sending process can be transmitted explicitly using a
 control message of type SCM_CREDS with a data portion of type struct
 cmsgcred, defined in <sys/socket.h> as follows:

 struct cmsgcred {
   pid_t cmcred_pid;       /* PID of sending process */
   uid_t cmcred_uid;       /* real UID of sending process */
   uid_t cmcred_euid;      /* effective UID of sending process */
   gid_t cmcred_gid;       /* real GID of sending process */
   short cmcred_ngroups;       /* number of groups */
   gid_t cmcred_groups[CMGROUP_MAX];     /* groups */
 };

 The sender should pass a zeroed buffer which will be filled in by the
 system.

 The group list is truncated to at most CMGROUP_MAX GIDs.